Ah yes. The speed at which the Universe is expanding is not a constant. This is a very interesting consequence of the presence of dark energy in our Universe, and gives us an interesting view into the very distant future of our Universe.
Dark energy is a descriptive term we have applied to a force which is responsible for the observed expansion of the Universe. We can see that the Universe is expanding, by measuring the apparent speeds of objects in the Universe, all of which appear to be receding at a rapid clip. Considering that we’re not at a special place in the Universe, this observation is best explained by every object drifting away from every other object. Given that gravity is also present in our Universe, some other force must be acting upon each and every object in our Universe in order to counteract gravity and keep them on their paths to an increasingly isolated Universe.
This force which counteracts gravity has been dubbed Dark Energy, and exactly what it is and how it operates is still extremely poorly understood. However, based on our observations, it must make up about 68% of all the energy present in the Universe to be able to do what we observe it doing - pushing all galaxies which aren’t tied to each other by gravity further apart from each other.
It’s one thing to have an expanding Universe. If the expansion occurred at a fixed rate, we would draw our Universe expanding as a straight line in diagrams like the one above. However, we have found that the expansion of our universe is happening at an increasingly rapid pace. Our Universe’s expansion is accelerating, not constant. This means that if I watch two galaxies separating now, from some kind of Universal bird’s eye view, and then came back in a billion years, and watched another set of two galaxies expanding away from each other, the second set would separate much faster than the first set.
Over a long enough period of time, this increasing speed of expansion means that the density of objects within the Universe will decrease. If every galaxy is increasingly distant from every other galaxy, images of galaxies outside our own Milky Way will also become increasingly out of date, as the light travel time also increases. If we pursue the increasing isolation of galaxies to its logical extreme, we arrive at an end-of-Universe scenario called “heat death”. Heat death arrives when a galaxy runs out of gas to form new stars, and the stars which remain are overwhelmingly either very faint red, brown, and black dwarf stars, black holes, or neutron stars. With no new gas able to arrive into the galaxy, the galaxy must end its star formation. Once the remaining red dwarfs and other stellar objects radiate away the last of their heat, and the entire Universe has reached a single, even temperature, we have arrived at the death of heat in our Universe. This is currently our Universe’s forecast for its eventual end state - and a direct consequence of having such a large amount of Dark Energy, pressing our Universe outwards into an ever-faster expansion.
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